You push off your propellant. In order to make your ship move, you expell superheated gas out the back at extremely high velocity. Basically, you bring the stuff you push off of with you.

By analogy, if you're standing on a skateboard and throw a ball, you'll move in the direction opposite your throw. You may be resting on the ground, but you didn't push off of the ground. Rather, you pushed off the ball.